Add MIT license file

[mtmail]

The exception_handler.gemspec file lists the MIT license. It's my understanding the license itself requires a copy to be distributed with the work.

Fixes #92

[coveralls]

Pull Request Test Coverage Report for Build 1564

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage increased (+0.2%) to 62.044%

---

Files with Coverage Reduction	New Missed Lines	%
lib/exception_handler/config.rb	1	77.78%

Totals
Change from base Build 1563:	0.2%
Covered Lines:	85
Relevant Lines:	137

---

💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 1564

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage increased (+0.2%) to 62.044%

---

Files with Coverage Reduction	New Missed Lines	%
lib/exception_handler/config.rb	1	77.78%

Totals
Change from base Build 1563:	0.2%
Covered Lines:	85
Relevant Lines:	137

---

💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 1564

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage increased (+0.2%) to 62.044%

---

Files with Coverage Reduction	New Missed Lines	%
lib/exception_handler/config.rb	1	77.78%

Totals
Change from base Build 1563:	0.2%
Covered Lines:	85
Relevant Lines:	137

---

💛 - Coveralls