Improvements for uploading multiple files

[hannes-ucsc]

• implement "s3am upload-dir" (it should skip files that already exist unless --overwrite is passed)
• foo vs foo/ vs foo/prefix*
• add progress information
• implement "s3am generate-key FILE" to generate a master key
• compute etag locally for validation

[hannes-ucsc]

• we'll punt on upload-dir since Unix find can do that but we'll add --overwrite (#30).
• progress info can be achieved with --verbose
• s3am generate-key FILE is tracked as (#33)

[hannes-ucsc]

• foo vs foo/ vs foo/prefix* will be fixed (#29 and #27)

[hannes-ucsc]

• compute etag locally for validation tracked separately (#34)