Add API to filter named gems from backtraces.

#1554.
Make `filter_gem` work when the gem is vendored under the working dir.
Add cuke demonstrating usage of new gem filtering API.
handle cases where the exception is not encoded as UTF-8
fix for 1.8.7
spec file encoding declaration for 1.9.2 / 1.9.3
changelog for #1681
[skip ci]
Merge pull request #1682 from rspec/add-filter-gem-api

Add API to filter named gems from backtraces.
Merge pull request #1681 from rspec/handle_ascii8bit_encoded_exceptions

Handle cases where an exception is not encoded as UTF-8
Extract sandboxing logic into a separate file.

The formatting specs rely on a particular expected
backtrace, which depends on which lines in particular
bits of the sandboxing code are on. Before, changes
to `spec_helper` would require updating the formatting
support expected backtraces. This change will isolate it.
Address ruby warnings from spec files.

For rspec/rspec-support#106.