Add missing require of StringScanner
Restrict :section lockfile token to start of line

This fixes the problem of the wrong token type being returned for
uppercase gem names.  It turns out my leading whitespace comment was
incorrect.
Fix line, column positions in Lockfile

They were inverted in messaging which is very confusing.
Add Resolver::LockSpecification

This reconstructs a Gem::Specification from the information in the
lockfile.
Allow Lockfile#get to expect multiple types
Switch to LockSpecification#add_dependency

Dependencies in a lockfile are encountered one at a time, so
LockSpecification#dependencies= is not helpful.
Parse dependencies in a lockfile GEM section

These were not parsed previously which lead to crashes.  Now they are
parsed, but the work is not complete.  Notably dependencies without
version specifications (implied >= 0) are not handled correctly.

Part of #727
Parse dependency with default requirement

When a spec has a dependency with the default requirement the
requirement is not shown in the lockfile.  The lockfile parser ignored
such dependencies, but this no longer occurs.

Part of #727
Parse multiple requirements in a lockfile

Lockfile dependencies can have multiple requirements but this was not
handled.  A new token (,) has been added to parse this properly.

Part of #727
Use "line" and "column" in bug exception"
Improve test name

This disambiguates from parsing dependencies in the DEPENDENCIES section
Parse requirements in DEPDENCIES lockfile section

Previously the lockfile parser did not handle requirements on
dependencies in the DEPENDENCIES section.  Now it does.

Part of #727
Allow duplicate gems from the same source

Bundler allows you to specify the same gem twice for a single source.

Part of #727
Add GitSpecification#add_git_spec

This creates a GitSpecification from the arguments as the Lockfile has
sufficient information for the creation.
Parse git sources and dependencies from a lockfile

This adds missing support for git gems in the Lockfile.

Part of #727