Exec: add option for ruby 1.9 FD behavior for 2.0

Ruby 2.0 changed the behavior of file descriptors when doing a Kernel.exec.
Where in 1.9 it wouldn't automatically close non-standard file descriptors
when doing a Kernel.exec, in 2.0 the default is to close them.

Issue #2628
add some documentation, warn if passing --keep-file-descriptors on < 2.0
only invalid gemname

white space fixes

add a line
Merge pull request #2630 from jendiamond/invalid_gemname

Invalid gemname
don't require what we don't use
Merge pull request #2629 from steved555/master

Exec: add option for ruby 1.9 FD behavior for 2.0
exec flags must go before the command
reword exec docs for clarity
no need to insert a dep in seen list if its already there, no need to double allocate the array.
Merge pull request #2631 from SamSaffron/master

perf: no need to insert a dep in seen list if its already there.
gemspec template: clarify summary vs description.

I never remember which is which, so I clarified based on http://guides.rubygems.org/specification-reference/.
Merge pull request #2635 from henrik/patch-1

gemspec template: clarify summary vs description.
Print a better error when git is not installed.