Multiple stable coins support in USD Tiered STO

[maxsam4]

Please check if the PR fulfills these requirements

• The commit message follows our Submission guidelines
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

What kind of change does this PR introduce?

(Bug fix, feature, docs update, ...)
Added Multiple stable coins support in USD Tiered STO

Does this PR introduce a breaking change?

(What changes might users need to make in their application due to this PR?)

1. An array of stable coin addresses has to be passed now rather than a single stable coin when configuring.
2. The investor has to provide the stable coin address they want to use when buying tokens with USD.

[adamdossa]

@maxsam4 Only minor comment is you could consider combining validSC and validUSDToken as I think they'll always be called together.

Otherwise looks good to me, with the caveat that the dApp / CLI will need some minor modifications for this.

[maxsam4]

Thanks @adamdossa
Pushed commit to merge the modifiers. My coding style is still one function one task but that does not work with solidity. Adapting is hard :).

[coveralls]

Coverage increased (+0.005%) to 98.035% when pulling 15d9dcf on multiple-stable-coins into 2336d85 on dev-2.1.0.