This allows callbacks to be combined with oneofs and pointer fields (#175)

This seems like it will result in less boilerplate in typical situations
where a specific message is either decoded or encoded. Otherwise it would
result in lots of dummy functions like:

bool MyQuery_encode_callback(...)
{
 return false; /* MyQuery is never encoded */
}

Now field information is stored as a variable-length uint32_t array.
This reduces the minimum size in most cases:

                            old            new
basic field:                10 bytes       4 bytes
repeated field:             10 bytes       8 bytes
field with 12bit tag/size:  16 bytes       8 bytes
field with 16bit tag/size:  16 bytes       16 bytes
field with 32bit tag/size:  28 bytes       16 bytes

Further, now the descriptor size can be increased per-field and per-message,
instead of previous global PB_FIELD_16BIT and PB_FIELD_32BIT.

PB_FIELD_32BIT still affects the pb_size_t type, which is 16-bit by default.

This will enable further development of the field descriptor encoding.

This makes nanopb behaviour match protoc --cpp_out=.

The timestamp causes unnecessary trouble when storing generated
files in version control.

Clarify the usage of callback fields inside oneof sections in the docs

cmake: Include CMAKE_INSTALL_LIBDIR in CMAKE_INSTALL_CMAKEDIR

cmake: Allow to build a shared library

If user defines non-standard directory in CMAKE_INSTALL_LIBDIR
option (i.e. /usr/lib64), then CMAKE_INSTALL_CMAKEDIR should
use it as a prefix.

Introduce options BUILD_SHARED_LIBS and BUILD_STATIC_LIBS to allow
building both types of libraries. By default only BUILD_STATIC_LIBS
is enabled.

Made some changes for Error Output.

Sometimes python protobuf version can't be found out by the above listed command. So added one more command to find python protobuf version which utilises pip freeze command.

Update README.md w/minor formatting

Add minor formatting marks to make more readable. Ex: `` for `code` and ** for *italics*.