Save speechAssets

[Glogo]

• add speechAssets.saveToDirectory(directory) function that saves speech assets files into specified directory
• example: speechAssets.saveToDirectory('speechAssets') - which is default value when param is ommitted
• add speechAssets directory to .gitignore
• /speechAssets directory should have following structure:

.
├── intentSchema.json # With the content of speechAssets.intentSchema
├── utterances.txt   # With the content of speechAssets.utterances
└── customSlots
    ├── customSlotName1.txt # With the content of customSlotName1 list of sample utterances
    └── customSlotName2.txt

• update speechAssets.toString() to be more readable

[Glogo]

thx @mmacai pull #10