Follow suggestions from rOpensci reviews

[AliciaSchep]

• [x] Add statement of need to README
• [x] Add reference to BiocInstaller in installation guidance
• [x] Add examples for setup_colorbar_grid and save_iheatmap
• [x] Add DOI to references in paper.md
• [x] Add Travis for Mac
• [x] Add App-veyor for Windows
• [x] Add badge in readme for minimum R
• [x] Explain better the differences between clustering vs clusters vs groups in Vignette
• [x] Explain better differences between clustering and dendrogram functions in Vignette
• [ ] Fix warning in automated tests?
• [x] Update description to not start with package name
• [x] Add contributing guidelines