Add build status to the README

Also make some general improvements to sections.
README update

Make the build status image link to the Travis project instead of the image.
Support installing packages from sources

In DESCRIPTION:
Depends:
    foo,
    bar,
    baz
Sources:
    foo (git=git://github.com/bar/foo.git, branch=working),
    bar (url=http://my.package.repo.com/bar.tar.gz),
    baz (file=/app/vendor/baz.tar.gz)

Refactored parse_sources based on code review

Pulled parse_sources into its own file
Added more descriptive variable names
Added unit tests

Refactored install_from_XXX based on code review

improved variable names
use @importFrom to import functions from dev_tools
moved install_from_XXX to separate file install_from.r
adding documentation to install_from_XXX

Added unit tests for install_from

Refactoring based on code review 3/21/2014

test_double documentation

Changed name of double to test_double
to avoid name conflict with double in package:base

Added documentation to function test_double

Removed quotes from list key names

DRYed up test_parse_sources

Interface takes a string for packages instead of the package
retrieved from mock_packages[[package]]

Cleaned up indentation in parse_sources.r

Added sources_empty function

Makes check for whether sources field in DESCRIPTION exists easier to read

Refactored parse_sources for readability

Moved all regexes into helper functions so that main function is more self documenting

Cleaned up indentation in test-bundler.r

DRYed up test_install_from

invocation of install_from_repos_or_sources

Matches the function signature

Throw error if source not found

Use https:// instead for git:// for tests

When hitting an external git URL, travis-ci needs https:// to work
Merge pull request #29 from KentonWhite/sources

Support installing packages from sources