Allow building and installing pulsevideo client and server seperately

They have different dependencies, so this makes it easier to build
and install just the client.
Allow specifying caps to use on the command line

... rather than hard-coding them to 720p30 RGB.

In the future it would be good to derive the caps to use from the pipeline
but this will require further work.
gitignore: Fix name of libgstpulsevideo.so

It was changed from gstpulsevideo.so to libgstpulsevideo.so in 222021b
but .gitignore was not updated.
Travis: Fix install command