* Add new file option php_namespace.

Use this option to change the namespace of php generated classes.
Default is empty. When this option is empty, the package name will be
used for determining the namespace.

* Uncomment commented tests

* Revert gdb test change

* Update csharp descriptor.

* Add test for empty php_namespace.

Fix the bug by sharing the code for generating class name for both
message and enum.

This option will be prepended to generated classes of all messages in
the containing file.

This better shows the semantic of the API. For already setted fields,
mergeFromString do replacement for singular fields and appending for
repeated fields.

This method merges the contents of the specified message into the
current message. Singular fields that are set in the specified message
overwrite the corresponding fields in the current message.  Repeated
fields are appended. Map fields key-value pairs are overritten.
Singular/Oneof sub-messages are recursively merged. All overritten
sub-messages are deep-copied.

1. Generate single file for each message.
2. Lazily initiate metadata.

This pull request includes two implementation: C extension and PHP
package. Both implementations support encode/decode of singular,
repeated and map fields.