add test suite using capistrano-spec (#87)

This is very far from complete coverage!  But starting is half the
battle, and the first few specs should serve as good examples for how
to write others.

A bug with rvm:empty_gemset requiring sudo was also fixed.
factor out rvm_system_path and rvm_user_path, and fix minor path bugs

This removes duplication of some hardcoded values and paves the way
for adding support for mixed mode installations.

Remove trailing '/' from "$HOME/.rvm/", for consistency with default
system path "/usr/local/rvm".

Finally, in the corner case that rvm_type is set to a non-standard value,
rvm_bin_path will end up being the bin/ subdirectory of rvm_path,
rather than the same directory.
make default value of rvm_gemset_path depend on rvm_type

This paves the way for adding support for mixed mode installations.
install_ruby.rb: remove duplicated code
add support for mixed installation mode via new :rvm_user parameter

rvm_user parameter will trigger running 'rvm user' with appropriate
arguments on the server.  It also helps determine the correct path for
importing/exporting gemsets, and when to use sudo (e.g. for gemset
creation/deletion only if rvm_user includes :gemsets).
Merge pull request #86 from aspiers/rvm-user

add proper support for mixed mode
1.5.0.rc1
Allow overriding the install url.

I am behind a proxy that for some reason does not guess that 'get.rvm.io' should
be redirected to 'https://get.rvm.io'. With this solution I can explicitely
declare the https url.
Another solution would be to hard code the protocol "https://" within the
variable command_fetch.
Fix default rvm url
Merge pull request #90 from real-chocopanda/allow-choosing-http-protocol

Allow overriding the install url.
clarify rvm:install_rvm does install/update
Merge branch 'master' of https://github.com/wayneeseguin/rvm-capistrano
make 'rake' run rspec tests
remove superfluous parameter in call to run_without_rvm
Merge pull request #91 from aspiers/fix-run_silent_curl

remove superfluous parameter in call to run_without_rvm
add rvm:info_list
1.5.0
Merge branch 'master' of https://github.com/wayneeseguin/rvm-capistrano
restrict capistrano version to 2.15.x as 3.0 was released and is not yet tested, fix #94
1.5.1