Package management additions

[drewbanin]

This PR refactors some of the in-progress dbt deps work and handles errors a little nicer.

Packages are extracted to the correct directory:

$ tree dbt_modules/ -L 1
dbt_modules/
├── dbt-utils
├── downloads
└── redshift

Invalid version specified for a dep:

$ dbt deps
Encountered an error:
Could not find a matching version for package fishtown-analytics/dbt-utils
  Requested range: =2.0.0, =2.0.0
  Available versions: ['1.0.0', '0.9.0']

Conflicting versions are specified:

$ dbt deps
Encountered an error:
Could not find a satisfactory version of fishtown-analytics/dbt-utils:
Specified versions: [>1.0.0, <1.0.0]

Nonexistent package is specified:

$ dbt deps
Encountered an error:
Package fishtown-analytics/snowflake was not found in the package index

TODO:

• Extract to correct dir
• Refactor deps task
• Nice error messages
• Work with local packages
• Work with remote repo (github, bitbucket, etc)
• Recursively clone deps for non-hub packages?
• Specify a required dbt version range in packages

[drewbanin]

TODO: Merge back into #542