Fix a few casts where ints were cast to uints unnecessarily

Fixes #199. Previously, enums were treated as uint32's, but they need to
be treated as int32's instead.

t: Add a few test cases with negative enum values

Add support for multiple versions of GCC and clang by using a build
matrix. Switch Travis VM to be based on Ubuntu 14.04

Adds support for the LITE_RUNTIME optimization option to the protobuf-c
compiler. Enabling this option would generate lighter weight message,
enum, and service descriptors that contain NO strings. As a result,
calls to lookup descriptors via the *_get_{field,value,method}_by_name
API will return NULL.

Default compiler behavior (when optimize_for is not specified or is not
set to LITE_RUNTIME) is unchanged.

Certain protobuf comments could generate invalid C comments
and inadvertently close the comment block. This commit removes '/'
signs in such comments.

One example of a .proto file containing such comments is a commonly
included descriptor.proto from the protobuf library.

Check for NULL pointer for repeated fields passed to protobuf_c_message_free_unpacked

Closes #177

Check for NULL message pointer passed to protobuf_c_message_free_unpacked

Closes #177

warning: assigning to 'uint8_t *' (aka 'unsigned char *')
from 'char *' converts between pointers to integer types
with different sign [-Wpointer-sign]

Add test cases for parsing multiple fields of the same oneof from the wire
Add more tests for submessage merging, including oneofs

Add a field flag and functions to pack/unpack oneofs
Add logic to parse multiple fields for the same oneof
Add logic for merging submessages that contain oneofs