add option parsing unit test
add opt specs

opt_specs list contains all valid command line options, different
option argument types are available, arguments can be either
non-existing, strngs, tuples or numbers
replaced argument processing methods with new ones that
make use of opt_specs
add protoc-erl command line tool

accepts all currently available gpb options
fix rebar gpb_compile invocation
fix rebar gpb_compile invocation
Merge branch 'add_protoc_cmdline_tool' of https://github.com/lrascao/gpb into add_protoc_cmdline_tool

Conflicts:
	rebar.config
update README.md with protoc-erl invocation
make protoc-erl executable
fix indentation, lines that exceed 80 columns
remove trailing whitespace
fix ebin string directory construction for code loading
fix protoc-erl status code when error occurs
fix handling of help and version options
fix defective indentation and spacing
fix handling of wrongly supplied options
fix options that have no arguments
Merge branch 'lr/add_protoc_cmdline_tool'
Prepare for 3.13.0