Add __virtual__ functions for pecl and pip states

Similar to what was recently done for npm, disable pecl and pip states
when the needed binary is not available. This prevents tracebacks when
one tries running these states without the required shell command
installed.
Better documentation for npm, pecl, and pip states

This commit adds additional info to the top docstring for these states,
including SLS examples for how to properly set up requisites to ensure
that npm/pecl/pip is installed before these states are run.
Merge pull request #6147 from terminalmage/quickfix

Add __virtual__ functions for pecl and pip states