Add basic support for mechanical transmissions.

- Base transmission class with abstract interface.

- Specializations for three common transmission types: simple, differential and
  four-bar-linkage.

- Unit tests with exercising preconditions, black-box and white-box tests.
Merge branch 'master' of github.com:willowgarage/ros_control into transmission_interface
Document abstract Transmission class.
Basic documentation for implemented transmissions.
Add comprehensive doc to implemented transmissions.

- More desriptive overview.

- Images depicting each transmission type. Binary pngs  are under version control
  instead of getting auto-generated in the Makefile as not all build environments
  may have the necessary svg->png filters.

- Expressions governing transmissions in tabular form.
Merge branch 'master' of github.com:willowgarage/ros_control into transmission_interface
Merge pull request #26 from jbohren-forks/master

Adding explicit header for recursive mutex
Merge branch 'master' of github.com:willowgarage/ros_control into transmission_interface
Add credit statement in docs.
Remove unnecessary virtual keywords.
Fixing error message in JointCommandInterface
Merge pull request #27 from jbohren-forks/fix-joint-cmd-interface-error-msg

Fixing error message in JointCommandInterface
Add transmission accessors test.
Add transmission interface class and test.
Remove dependency from manifest.
Add pthread dependency to tests.

After moving from Ubuntu 10.04 to 12.04 these dependencies need to be explicitly
stated in my dev machine. This should be looked upon in greater detail, as such
dependecies should be taken care of by rosbuild.
Use \name commands in documentation.
Remove pure virtual method.
Add readme file.
Update transmission_interface/README.md