Allow user to override the agent (curl or wget) when installing.

This fixes #80.  However, it only affects how the installer fetches
cpanm.  cpanm itself has it's own logic for deciding which agent to
use.  You can control this using the PERL_CPANM_OPT variable.
Document configurable variables.
Merge branch 'master' of github.com:thaljef/Pinto